1. Field of the Invention
The present invention relates to a Liquid Crystal Display device, and particularly, to a Liquid Crystal Display device which is capable of removing an inferior color split phenomenon at outer portions and edge portions of a light guide plate by mixing desired colors in the light guide plate and exiting light resulting from disposing lateral surfaces of the light guide plate and ends of light emission surfaces of light emitting devices disposed at both ends of a light emitting device array to be consistent with each other.
2. Discussion of the Related Art
Recently, a Cathode Ray Tube (CRT), one of display devices generally used, is mainly used for monitors of a television, a measuring apparatus, information terminal apparatus, etc. However, the CRT cannot be actively applied to electronic products having a small size and a light weight due to the CRT's weight and size. Thus, the CRT has a limitation in following a trend that various electronic products become small and light. As a substitute the CRT, there are a Liquid Crystal Display (LCD) device using an optical effect of an electric field, a Plasma Display Panel (PDP) using an electric discharge of gas, and an Electro Luminescence Display (ELD) device using a light-emitting effect of an electric field. Among these display devices, the LCD device is being actively researched.
In order to substitute the CRT, the LCD device having advantages such as a small size, a light weight and a low power consumption has been developed to be sufficient to serve as a flat panel display device, recently. Thus, the LCD device is used for a monitor of a desktop computer and as a large-sized information display device, and as a display device for many other purposes. Accordingly, the demand for the LCD device is continuously increasing.
The LCD device can be largely divided into a liquid crystal display panel for displaying images and a driving unit for applying driving signals to the liquid crystal display panel. The liquid crystal display panel includes first and second glass substrates combined with each other with a predetermined space therebetween and a liquid crystal layer infused between the first and second glass substrates.
Meanwhile, since the LCD device is mostly implemented as a light-receiving device displaying images by adjusting the amount of a light source, it is necessary to have a separate light source for irradiating a light onto the liquid crystal panel, which is a backlight. The backlight is classified into an edge type and a direct type according to a position where a lamp unit is installed.
The light source may be implemented as an Electro Luminescence (EL), a Light Emitting Diode (LED), a Cold Cathode Fluorescent Lamp (CCFL), an External Electrode Fluorescent Lamp (EEFL), and etc. Among these light sources, the CCFL type is actively used for a large screen color TFT LCD because it can be configured to have a long lifespan and a low power consumption and to be thin. The CCFL type uses a fluorescent discharge tube in which Mercury (Hg) gas with Argon (Ar), Neon (Ne), and etc. added to the Mercury gas is filled to be sealed in a low pressure so as to implement a penning effect.
Electrodes are formed at both ends of the discharge tube. A cathode is formed in a plate shape. When a voltage is applied, a charged particle in the discharge tube collides with the plate-shaped cathode and thus a secondary electron is generated, like a sputtering phenomenon. Accordingly, peripheral elements are excited, thus forming plasma. This causes the peripheral elements to radiate a strong ultraviolet ray. Then, the ultraviolet ray excites a fluorescent substance, and accordingly the fluorescent substance radiates a visual ray.
The aforementioned edge type means that a lamp unit is installed at a lateral surface of the light guide plate guiding light. The lamp unit is provided with a lamp radiating light, a lamp holder protecting the lamp by being inserted into both ends of the lamp and a lamp reflection plate for reflecting the light radiated from the lamp toward the light guide plate by encompassing an outer circumferential surface of the lamp and having one lateral surface inserted into the lateral surface of the light guide plate. The edge type where the lamp unit is installed at the lateral surface of the light guide plate is generally applied to a relatively small LCD device such as monitors of a laptop computer and a desktop computer. It has advantages that light is uniformly radiated, an endurance life is long and the LCD device can be easily formed in a thin film.
According to the edge type backlight unit, light radiated from the florescent lamp is collected on a light incidence surface of the light guide plate and then transferred to the liquid crystal display panel sequentially through the light guide plate, a diffusion plate and a prism sheet. However, the backlight unit using a related art florescent lamp has a low color reproduction range due to a light-emitting characteristic of the light source itself. It is difficult to obtain a backlight unit having a high brightness due to limitations on a size and a capacity of the fluorescent lamp.
Meanwhile, the direct type that is different from the edge type in aspects of a position and an arrangement of the light source has been actively developed as the LCD device having a large size (for example, more than 20 inches) started to be developed. The direct type is implemented by arranging a plurality of lamps in a row on the lower surface of the diffusion plate and then directly irradiating light toward the front surface of the liquid crystal panel. Since the direct type backlight unit has a utilization efficiency of light higher than the edge type backlight unit, it is mainly used for a large screen LCD device requiring a high brightness.
The aforementioned edge type and direct type backlight units use the fluorescent lamp as the light source. However, researches on a new light source are performed due to the recent concern about harmful gas filled in the fluorescent lamp, recently. Among the available light sources, the LED is focused as the new light source because it does not cause an environmental pollution, it can implement various colors and it can reduce the power consumption.
Hereafter, the related art LCD device using the LED light source will be explained with reference to FIGS. 1 to 5.
FIG. 1 is a section view schematically showing a LCD device having a backlight unit in accordance with the related art.
FIG. 2 is a planar view schematically showing an arrangement structure of a LED array and a light guide plate in accordance with the related art.
FIG. 3 is a view schematically showing the LED array in accordance with the related art.
FIG. 4 is a view schematically showing the LED array and the light guide plate in accordance with the related art.
FIG. 5 is a front view schematically showing that an inferior color split phenomenon occurs when light is incident onto the light guide plate under a state that the light guide plate and the LED array are spaced from each other by a predetermined distance (D), in the LCD device in accordance with the related art.
The LCD device in accordance with the related art, as shown in FIG. 1, includes a liquid crystal display panel 70 displaying images and a backlight unit 10 emitting light toward the liquid crystal display panel 70. Here, in the liquid crystal display panel 70, though it is not shown, liquid crystal is filled between an upper substrate and a lower substrate. Further, the liquid crystal display panel 70 is provided with a spacer for maintaining a uniform gap between the upper substrate and the lower substrate. The upper substrate of the liquid crystal display panel 70 is provided with a color filter, a common electrode, a black matrix, etc. The lower substrate of the liquid crystal display panel 70 is provided with signal lines such as a data line and a gate line. In addition, a Thin Film Transistor (TFT) is formed at an intersecting portion of the data line and the gate line. The TFT serves to convert a data signal to be transferred to a liquid crystal cell from the data line in response to a scan signal (gate pulse) from the gate line. Further, a pixel electrode is formed at a pixel area between the data line and the gate line. An upper polarizing plate is attached on the upper substrate of the liquid crystal display panel and a lower polarizing plate is attached on the lower substrate thereof.
Meanwhile, the backlight unit 10, as shown in FIGS. 1 and 2, includes a light guide plate 30 disposed to face the liquid crystal display panel 70, a reflection plate 20 disposed at a lower side of a LED array. The LED array includes a plurality of LEDs 11 composed of LEDs 11R, 11G, 11B of red, green and blue colors respectively disposed to face one lateral surface of the light guide plate 30. Further, the backlight unit 10 also includes an optical sheet 60 disposed between the light guide plate 30 and the liquid crystal display panel 70. The backlight unit 10 is inserted by fixing a support main 17 with a lower cover 21.
The optical sheet 60 is divided into a vertical optical sheet 61 and a horizontal optical sheet 62. Here, the optical sheet 60 serves to increase a front brightness of light passing through the optical sheet 60. That is, the optical sheet 60 is configured to transmit light only by a specific angle. Moreover, the light incident by other angles is reflected by an internal total reflection in the optical sheet 60 and thus returns to a lower portion of the optical sheet. Then, the returning light is reflected by the reflection plate 20.
The LED array composed of the plurality of LEDs 11, as shown in FIGS. 2-4, is fixed on a Package Circuit Board (PCB) 13 formed of a metallic material, and the PCB 13 is installed in the support main 17. Further, a connector 80 is provided at an edge portion of an upper surface of the PCB 13 so as to electrically connect each LED 11 of the LED array to an external driving circuit through a cable 15.
In addition, light guide plate stoppers 19 are provided at both edge portions inside of the support main 17 so as to protect the LEDs 11 and the light guide plate 30 from vibration of a product and prevent the LEDs from being damaged by contact between the LEDs 11 and the light guide plate 30. Here, the light guide plate stoppers 19 are disposed at both sides inside of the support main 17 corresponding to both edge portions of the light guide plate 30 so as to contact the both edge portions of the light guide plate 30 and thus support the same. The light guide plate stoppers 19 are slightly higher than the LEDs 11 so as to contact and support the light guide plate 30, while maintaining some distance between the LEDs 11 and the light guide plate 30. Accordingly, it is capable of preventing the light guide plate 30 from directly contacting the LED array due to vibration of the product.
Here, the LED array, as shown in FIGS. 1 and 2, is disposed to be spaced apart from a light incidence surface 30a of the light guide plate 30 by a predetermined distance (D) corresponding thereto. Here, the red LED 11R and the blue LED 11B disposed at the outer most portion of the both edge portions of the LED array are inwardly spaced from both side ends of the light incidence surface 30a of the light guide plate 30 by a predetermined distance. Further, the connector 80, as shown in FIGS. 3 and 4, is disposed at a position corresponding to one edge portion of the light incidence surface 30a adjacent to the lateral surface of the light guide plate 30, that is, at one edge portion of the upper surface of the PCB 13.
However, the LCD device in accordance with the related art has the following problems. In the LCD device according to the related art, the emitted light cannot be implemented in white, but can only be implemented in other colors. That is, a red light is emitted at a left end of the light guide plate by the red LED and a blue light is emitted at a right end of the light guide plate by the blue LED, as shown in FIG. 5 as “A” and “B”, depending on a wavelength and a light intensity of the R, G, B or arrangement positions and a sequence of the R, G, B LED array, when applying the R, G, B LEDs. Since the light guide plate and the LED array are spaced from each other by a predetermined gap therebetween (e.g., “D” of FIG. 1), the red light is emitted at the left end of the light guide plate and the blue light is emitted at the right end of the light guide plate. Particularly, a desired white light is not reproduced at the end of a light incidence portion of the light guide plate due to a color split phenomenon at the edge portions of the LCD device, which are both ends of the light guide plate.
The reason why the color split phenomenon occurs is as follows. Since the light emitted from a LED disposed at the end of the LED array is relatively more intensive than the light emitted from an adjacent LED, the desired white light is not reproduced at the end of the light incidence portion of the light guide plate by the LEDs implementing other colors even when a power is precisely applied to each LED.
Meanwhile, in the LCD device in accordance with the related art, power consumption and a heat generation increase because the number of LEDs in the related art is higher than when white LEDs are used instead. In order to control the increase of the heat generation, a metallic PCB is used for the LCD device. When the metallic PCB is used the LCD device, additional cable and connector are required to connect the LED driving unit.
Thus, the LEDs cannot be disposed at one edge portion of an upper surface of the metallic PCB on which the LED array is disposed. Further, since the cable and connector are disposed at the edge portion of the upper surface of the metallic PCB, there is decreased available space on the metallic PCB for the LEDs to be disposed, and thus the number of LEDs that can be disposed on the metallic PCB may be decreased.
Furthermore, in order to prevent the LED array from being damaged by the contact between the LED array and the light guide plate, a light guide plate stopper (LGP stopper) should be additionally disposed at both edge portions in the support main, apart from the PCB on which the LED array is disposed. Accordingly, a structure of the LCD device may be complex and a fabrication cost for the LCD device may increase.